1. Field of the Invention
The present invention relates to a semiconductor ceramic having a negative resistance-temperature characteristic, and a semiconductor ceramic device including the same, and more particularly to a semiconductor ceramic especially for use in inrush current control of a switching power supply or the like, temperature compensation of a device such as a quartz oscillator or the like, motor start-up, and so forth, and a semiconductor ceramic device including the same.
2. Description of the Related Art
Conventionally, there are available semiconductor ceramic devices (hereinafter, referred to as an NTC device) having a negative resistance-temperature characteristic (hereinafter, referred to as a negative characteristic) in that the resistance at room temperature is high and decreases with the temperature being raised. The NTC devices are applied in various uses, for example, in temperature compensation type quartz oscillators, inrush current control, motor start-up retardation, halogen lamp protection, and so forth.
For example, temperature compensation type quartz oscillators (hereinafter, referred to as TCXO) used as a frequency source for electronic apparatuses such as communication equipment and so forth. comprise a temperature compensation circuit and a quartz oscillator. A temperature compensation type quartz oscillation device in which a temperature compensation circuit is connected directly to a quartz oscillator in an oscillation loop is called a "direct TCXO", and that in which a temperature compensation circuit is connected indirectly to a quartz oscillator out of an oscillation loop is called an "indirect TCXO".
The direct TCXO contains at least two NTC devices in order to temperature compensate the oscillation frequency of the quartz oscillator. One NTC device is used for temperature-compensation at room temperature (25.degree. C.) or lower and has a low resistance at room temperature of about 30 to 150 .OMEGA.. The other NTC device is used for temperature compensation at room temperature or higher and has a high resistance at room temperature of about 2000 to 3000 .OMEGA..
In switching power supplies and lighting circuits of halogen lamps, an eddy current flows the moment that a switch is turned on. To prevent the eddy current from flowing, an inrush current controlling NTC device is used for absorbing the inrush current generated in the initial stage. When the power supply switch is turned on, the NTC device absorbs the inrush current in the initial stage to control the eddy current which flows in the circuit. After this, the NTC device, whose temperature is raised due to self-heating, has a lower resistance, and in the stationary state, the power consumption is reduced.
Further, in a motor provided in a gear having a structure such that a lubricating oil starts to be fed therein after the motor is started up, it is preferred that the speed at which the gear is rotated is increased stepwise to a high speed by application of a current. Further, in a lapping machine with which the surface of porcelain is abraded by rotating a grindstone, preferably, a driving motor is started-up, and the lapping machine is rotated with the speed being increased stepwise to a high speed. As a device for retarding the rotation-start time of the gear or the grindstone for a predetermined time at the start-up of the motor, an NTC device for retarding the motor start-up is used. Since the NTC device exhibits a high resistance at the start-up, the motor terminal voltage is reduced so that the start-up of the motor is retarded. After this, the temperature of the NTC device is raised, due to self-heating, and the resistance becomes low. Then, the motor terminal voltage is increased so that the motor is started up. In the stationary state, the motor is normally rotated.
Conventionally, as semiconductor ceramics having a negative resistance-temperature characteristic and constituting these NTC devices, spinel oxides containing transition metal elements such as manganese, cobalt, nickel, copper and so forth have been used.
To temperature-compensate TCXO oscillation frequency at a high precision, it is desirable that the temperature dependency (hereinafter, referred to as B constant) of the resistance of an NTC device is high. In general, spinel oxides containing transition metal elements have a positive correlation between the resistivity at room temperature and the B constant. The higher the resistivity at room temperature, the higher the B constant. Accordingly, the spinel oxides containing transition metal elements are suitable as materials for NTC devices which are required to have a high resistance at room temperature and a high B constant, that is, as materials for NTC devices which are used for temperature compensation at room temperature or higher. However, the spinel oxides are unsuitable as materials for NTC devices which need to have a low resistance at room temperature or lower and a high B constant, namely, as materials for NTC devices which are used for temperature compensation at room temperature or lower. By forming the NTC device so as to have a lamination structure containing plural internal electrodes laminated therein, the resistance of the NTC device can be reduced even though for the NTC device, a material having a high resistivity is used. However, the lamination structure causes the static capacitance of the NTC device to increase. After all, it is difficult to obtain a satisfactory temperature compensation with high precision.
Moreover, when an NTC device is employed for inrush current control, it is necessary that the resistance of the NTC device becomes low in the temperature-rising state caused by the self-heating. Conventional spinel oxides when they are employed show a tendency that the lower the resistivity, the smaller the B constant. Accordingly, the resistance in the temperature rising state is not satisfactorily low. Accordingly, as a method of satisfactorily decreasing the resistance of an NTC device at a high temperature, increasing the area or thinning the thickness is employed when the NTC device has a plate shape, for example. However, increasing the area of the NTC device is contrary to the miniaturization of the device. Also, from the standpoint of maintaining the strength of the NTC device, the thickness of the NTC device can not be thinned extremely. The resistance of the NTC device, even though a material having a high resistivity and a high B constant is used for the NTC device, can be made low by forming the NTC device so as to have a lamination structure in which there are plural internal electrodes. However, since the distances between the opposing internal electrodes are short, an allowable eddy current could not be significantly increased.
It has been revealed by the studies by V. G. Bhide, D. S. Rajoria and others that oxides containing rare earth metal elements have a negative resistance temperature characteristic in which the resistance is decreased in the temperature rising state at a high temperature. The study by A. H. Wlacov and O. O. Shikerowa has shown that as to the characteristics of the LaCoO.sub.3 type NTC devices, the resistance of LaCoO.sub.3 is low than that of GdCoO.sub.3 in general.
However, the LaCoO.sub.3 type NTC devices have a low resistivity at room temperature but have a B constant of less than 2000 K. Accordingly, when a LaCoO.sub.3 type NTC device is used to control inrush current and the resistance of the LaCoO.sub.3 type NTC device is adjusted for controlling the inrush current, the power consumption during the stationary time is increased.
To solve this problem, the inventors have found that the B constant can be enhanced to be 4000 K or higher by addition of an oxide of Cr to a major component comprising a lanthanum cobalt type oxide as reported in Japanese Patent Application No. 9-208310. That is, by controlling the addition range of the oxide of Cr, the B constants at low and high temperatures can be individually controlled. Accordingly, by selecting materials containing the lanthanum cobalt type oxides as major components suitably depending on intended use, materials become available for those various uses, e.g., for control of an inrush current, motor start-up retardation, halogen lamp protection, or the like for where it is required to increase the B constants at high temperatures, and for uses such as TCXO or the like where it is required to enhance the B constants at low temperatures.
Further, when a material containing a lanthanum cobalt type oxide as a major component and an oxide of Cr added thereto is used as a material for a lamination type NTC device, the lamination type NTC device having as low a resistance as a conventional device can be obtained, even though the number of internal electrodes is decreased as compared with a conventional lamination type NTC device. Accordingly, the static capacitance of the lamination type NTC device can be reduced to be lower than that of the conventional device. Further, since the distances between the internal electrodes can be increased, the allowable eddy current can be increased compared with the conventional device.
Further, when a composition containing the lanthanum cobalt type oxide as a major component and an oxide of Cr added thereto is used for an NTC device for controlling an inrush current, the B constant at a high temperature can be enhanced to be 4500 K. However, the B constant at a low temperature presents a value of 4000 K or higher.
Further, since the composition containing the lanthanum cobalt type oxide as a major component and an oxide of Cr added thereto has a high relative dielectric constant, the static capacitance becomes high.